Phosphine detected in the clouds of Venus

The acidic clouds of Venus may not seem like the most promising environment to find alien organisms, but new research suggests that our nearest neighbor is host to mysterious chemical processes that could suggest the presence of life. A team led by Jane Greaves at Cardiff University have attributed a microwave signal detected in the Venusian atmosphere to the presence of phosphine, a small molecule associated with anaerobic microbial life on Earth (Nat. Astron. 2020, DOI: 10.1038/s41550-020-1174-4).

With surface temperatures above 470 °C and a dense, oxidizing atmosphere composed of carbon dioxide, nitrogen, and droplets of sulfuric acid, it’s hard to explain the presence of a highly reduced molecule like phosphine (PH₃) on Venus. But that lack of a geochemical explanation for its presence is what makes it a good biosignature, or chemical trace of life. “We always look for gases that don’t belong,” says Janusz Petkowski, an astrobiologist at the Massachusetts Institute of Technology and an author on the paper.

The microwave signal was detected by two different telescopes: the James Clerk Maxwell Telescope in Hawaii and the Atacama Large Millimeter Array in Chile. After ruling out other possible sources for the signal such as sulfur dioxide, the researchers attributed it to a rotational transition of phosphine at a concentration of around 20 ppb. Further, the gas was only detected in the mid-latitudes of the planet and not at the poles, which scientists’ current understanding of planetary physics cannot explain

“There is no known chemical or physical process that could produce phosphine on a rocky planet,” says Petkowski, who led a companion, non-peer-reviewed study evaluating potential origins of the gas (arXiv 2020, arXiv: 2009.06499). His team ruled out known photochemical and geochemical processes on Venus and concluded that something else is happening on the planet. “If it is not life, then we have to revisit our understanding of what the planet itself can do,” he says, “because it means that planets are much better chemists than we previously believed.”

Anthony Remijan, an astrochemist at the National Radio Astronomy Observatory, who was not involved in the study, says that the team did a “fantastic job” presenting their findings, but he remains skeptical. Scientists need more spectral data to verify that this signal comes from PH₃, Remijan says. “But it’s a good first step in that direction.” As for what could produce phosphine on Venus, he says “we need a better understanding of the fundamental chemistry forming these types of molecules” before calling it a biosignature.

Petkowski and his team plan to seek out further confirmation of phosphine on the planet, and hope that this study could inspire new missions to bring back samples of the Venusian atmosphere.

Dirk Schulze-Makuch, an astrobiologist at the Technical University Berlin, says that he is “quite excited” about the implications of phosphine on Venus. If there is life on Venus, the cloud decks are the only place it could be, he says. “There’s so much we don’t know and Venus is still largely an unknown planet for us.”.